OBJECTIVE: To determine the molecular structure of the paramyosin containing thick filament of Limulus striated muscle and develop a model which explains how the filament shortens. We have already demonstrated by several methods that during sarcomere shortening in this muscle not only do thick and thin filaments slide past each other, but the thick filament shortens. This will be accomplished by: 1. Optical diffraction of isolated thick filaments (myosin). 2. Optical diffraction of isolated filaments stripped of cortical myosin (paramyosin core) 3. Determining diameter and number of subfilaments along the thick filament from serial sections of the filaments. 4. Determining in vitro requirements for thick filament shortening. 5. Determining protein composition of isolated thick filaments by 2 dimensional SDS gel electrophoresis. 6. Comparing parameters of thick filament of Limulus telson muscles to Limulus leg muscle where filaments do not shorten.